Autonomous Exploration of Earth’s Largest Subterranean Lake and Implications for Robotic Lunar and Mars Lava Tube Exploration

Abstract:

In August of 2019 a 14 person international team was fielded to the Tsumeb district of northern Namibia to investigate the world’s largest known subterranean lake at Dragon’s Breath cave. The cave is developed in limestone karst 60 m beneath the Kalahari desert. The lake measures 230 m long, 120 m wide and up to 20 m between the lake and the bedrock ceiling. Divers using the latest technology reached a depth of 132 m in July of 2015. Beyond this limit an enormous tunnel descends at a 20 degree slope. During the past 10 years Stone Aerospace developed an autonomous hovering underwater vehicle named SUNFISH with unusual capabilities. In addition to traditional dead-reckoning navigation it utilizes 3D geometry around the vehicle as input to a real-time simultaneous localization and mapping (SLAM) engine. The SLAM solution augments the dead-reckoning pose solution via 3D geometric feature recognition. Maps are constructed over a 4π steradian view about the vehicle and updated at 20 Hz. SUNFISH uses behavior-based programming as opposed to scripted missions, since there is no a priori knowledge of the exploration target at the start of a mission. Exploration at Dragon’s Breath was programmed using only high level goal states (e.g. attempt to proceed 1 kilometer to the north; maintain 10 m standoff from obstacles, etc). A dedicated exploration behavior seeks to maximize new knowledge within the constraints described above. The SUNFISH vehicle is capable of rotation about any axis (yaw, pitch, roll) and this feature – together with a 480-beam, 120-degree swath multi-beam acoustic instrument – was utilized to complete a spherical scan about the vehicle. These data were then analyzed to predict the next move that would maximize new knowledge of the unknown environment. The vehicle would then traverse to the most remote point in the “new data” area and repeat the process. Using this behavior SUNFISH explored to a depth of 205 m in Dragon’s Breath in a tunnel measuring 175 m wide and 70 m high to its terminus half a kilometer from the lake surface. The underwater volume measured 2,235,275 m³. The Dragons Breath mission carries significant lessons for robotic exploration and science operations in lunar and Martian lava tubes, which will be similarly completely lacking in a priori knowledge, despite significant orbital reconnaissance assets.